A direct conversion receiver is a radio receiver converting a high frequency signal received by an antenna into a baseband signal, using a local oscillator signal which is the same in frequency as the high frequency signal. This technique contributes to manufacturing smaller, lighter, and further power-saving radio receivers. A problem is, however, that the technique causes a generation of a direct current (DC) offset unique to the circuit.
A typical counter measure to this problem is to provide a high path filter (HPF) in a signal path of a variable gain amplifier (VGA) to be controlled in an AGC circuit, so that a capacitor included in this HPF removes a DC component.
In such a period as a gain searching period, for a VGA, in which a large DC offset is likely to be generated, a typical technique involves setting a cut-off frequency of an HPF high to immediately mitigate the DC offset. In a period other than the gain searching period, the typical technique involves setting the cut-off frequency of the HPF low to improve accuracy in demodulation. (See Japanese Unexamined Patent Publication No. 2003-224488.)
In the above typical technique, as will be described later, a large DC offset could be generated in an output of the VGA when the cut-off frequency of the HPF is switched from high to low.
The present disclosure attempts to offer an AGC circuit and a radio receiver which reduce a generation of a DC offset in an output of a VGA.